Determination of transport levels of organic semiconductors by UPS and IPS

In this paper, we present a comprehensive discussion on the determination of transport levels of organic semiconductors using a combination of UV and inverse photoemission spectroscopies (UPS and IPS, respectively). We discuss the key question of how the spectra should be evaluated to obtain the correct values for the transport levels. Our evaluation results in much smaller exciton binding energies than generally assumed, ranging from 0.1 to 0.4 eV for CuPc, PTCDA and Alq3. Moreover, we find that polarization effects, vibrations, experimental resolution and inhomogeneous broadening contribute very little to the position and width of the broad photoelectron spectroscopy (PES) and IPS signals. Thus, we suggest a model that explains much of the broadening as a result of dynamic charge delocalization. We discuss our data in analogy to those obtained for inorganic semiconductors (IOSCs), which are presented in a separate paper.